Level 4 is fine for eggshell and satin on ordinary walls. Level 5 — a full skim over the whole surface — is what you want for flat and matt sheens, for large open walls, and anywhere with big windows or strong downlighting, because raking light shows every joint that has not been skimmed. If the board has only been taken to level 4 and you want a flat finish, we will quote the skim separately rather than paint it and let the joints appear later.